Processes for the production of sandwich elements for the fabrication of flat sheets have been known for some time. The sandwich construction is made up of a light and compression-resistant core with high-strength covering layers. This composite structure is formed by a PU reaction mixture which, when applied on both sides in a thermal compression molding process, can form an indissoluble bond. The inner core layer of the sandwich structure is preferably made up of cardboard with a honeycomb structure, which during the compression procedure acts as a spacer for the covering layers wetted with PU. The wetting of the sandwich covering layers is preferably affected by spraying. During the PU application by means of a mixing head the substrate carrier is arranged in the horizontal or preferably in the vertical position, since in this orientation a double-sided application can be carried out at the same time. The substrate carrier and/or the polyurethane mixing head may be robot-guided.
Three-dimensional molded articles can also be produced due to the combination of compression and shaping processes. The honeycomb core, which for sheet materials is built up over the whole surface to a uniform thickness a few tenths of a millimeter thick, is now compressed in partial regions to a small percentage of its original size. The shaping of the outer contour of the finished part is effected by a nipping-off of the sandwich structure by pinch edging in the shaping tool (mold), so that the structural part has closed outer edges after removal from the mold. In this shaping process, a three-dimensional structural part is obtained that has unlaminated visible surfaces and also unlaminated visible edges.